1. Field of the Invention
The present invention relates to a method of driving a matrix type liquid crystal display.
2. Description of the Prior Art
The demand for large-screen thin type visual displays has increased, particularly in the industries of information equipment, such as computers, video equipment, and television receivers. For driving a known matrix type liquid crystal display, a voltage averaging method is commonly employed in which an effective voltage applied to a pixel during a non-selection period is constant (for example, as described in Japanese Patent Laid-open Publications No. 50-68419 (1975) and No. 55-140889 (1980).
Such a conventional driving method will be described below referring to the accompanying drawings.
FIGS. 1a and 1b are schematic views showing a liquid crystal display, in which scanning electrodes Y1, Y2, Y3, Y4, Y5, Y6, . . . , YN coupled to a scanning line driver 100 and signal electrodes X1 and X2 coupled to a signal line driver 101 are arranged in a matrix form for constituting an array of pixels. FIG. 2a shows a waveform diagram of driving voltages applied for displaying a pattern illustrated in FIG. 1a (in which the hatched pixels are OFF and the remaining pixel are ON). Shown in FIG. 2a are the waveforms of the respective voltages; V.sub.Y2 applied to the scanning electrode Y2, V.sub.X1 and V.sub.X2 applied to the signal electrodes X1 and X2 respectively, and V.sub.111 and V.sub.112 (difference voltages between the scanning and signal electrode voltages) applied to pixels 111 and 112, respectively.
It is known that the amount of transmitted light across a twisted nematic liquid crystal panel, which is one of the most typical matrix liquid crystal panels, corresponds to an effective value of the voltage applied thereto. As illustrated in FIG. 2, denoting the scanning period is T, the number of scanning lines as N, the maximum voltage to be applied as V.sub.O, and the bias ratio as a, the effective voltages V.sub.NS and V.sub.S applied to the OFF and ON pixels, respectively, are expressed as: ##EQU1## Also, FIG. 2b is a waveform diagram of driving voltages for exhibiting the pattern of intermediate gradation display illustrated in FIG. 1b. Denoting the scanning period as T, the period for applying an ON voltage [0, V.sub.O ] during the scanning with a signal electrode voltage as T.sub.S, the period of applying an OFF voltage [(2/a)V.sub.O, (1-2/a)V.sub.O ] during the same as T.sub.NS, the number of scanning lines as N, the maximum voltage to be applied as V.sub.O, and the bias ratio as a, the effective voltage V applied to a pixel is expressed by: ##EQU2## (where m=T.sub.S /T). Hence, the intermediate gradation pattern can be displayed by changing the duration of the selection voltage application and the duration of the bias voltage application in accordance with the gradation level (with the equation (1) if M=0 and (2) if M=1).
However, in the conventional method, the actual voltage applied to a corresponding pixel is affected by the presence of electrode resistance and liquid crystal capacitance, and thus exhibits a distorted waveform represented by the dotted line of FIG. 2a or 2b. FIG. 3 illustrates the waveform of a voltage distorted by inversion of the voltage polarity during the non-selection period. Hence, V.sub.NS and V.sub.S are altered from the equations (1) and (2)and are instead represented as: ##EQU3## where a portion a of the distorted waveform, denoted by the dotted line in FIG. 3, is linearly approximated to a line b, and T is the time required for b to become (1/a)V.sub.O or -(1/a)V.sub.O and n is the number of voltage changes from (1/a)V.sub.O to -(1/a)V.sub.O and from -(1/a)V.sub.O to (1/a)V.sub.9 (with 0.ltoreq.n.ltoreq.N-1) in one field T.sub.F.
Similarly, the effective voltage V is now altered from the equation (3) to: ##EQU4## (where m=T.sub.S /T and 0.ltoreq.n.ltoreq.2N-2). As is understood from the above, the effective voltage which is applied to each of the pixels having a same light transmittance, as shown in FIG. 2a, is varied by the values of t and n, thus preventing uniformity in the display and also causing the amount of transmitting light across the pixel to change adversely. This phenomenon will be emphasized in the display of intermediate gradation pattern with pulse width modulation as explained in FIG. 2b. Accordingly, as a voltage applied to a corresponding pixel is biased due to the presence of electrode resistance and liquid crystal capacitance, its waveform which varies corresponding to a pattern to be displayed and will be distorted more or less. Then, while the driving voltage becomes higher in frequency as the quality of the display is enhanced, the distortion in the waveform cannot be disregarded in order to provide a uniform display.